The present invention relates to an image forming apparatus such as a printer, a facsimile, a copier, and an MFP (a multi function peripheral or product) using an electro-photography type. In particular, the present invention relates to a fixing device of the image forming apparatus.
In a conventional image forming apparatus such as a printer, a facsimile, a copier, and an MFP (a multi function peripheral or product) using an electro-photography type, a charging roller uniformly charges a surface of a photosensitive drum. Then, an exposure device such as an LED (Light Emitting Diode) exposes the surface of the photosensitive drum to form a static latent image thereon. In the next step, a thin layer of toner formed on a developing roller is attached to the static latent image to form a toner image on the photosensitive drum. Afterward, the toner image formed on the photosensitive drum is transferred to a print medium supplied from a sheet supply device.
In the next step, after the toner image is transferred to the print medium, the print medium is transported to a fixing device. In the fixing device, heat and pressure are applied to the print medium, so that the toner image is fixed to the print medium, thereby forming an image on the print medium.
In the conventional image forming apparatus, the fixing device may be of a film heating type, in which a film is used as a rotational member. In the fixing device of the film heating type, the print medium is closely contacted with a heater through a heat resistant film. Further, a pressing roller is disposed as a pressing member to press against the heat resistant film. When the pressing roller rotates, the heater and the heat resistant film move relatively, thereby transporting the print medium. During the process, heat of the heater is conducted to toner and the print medium through the heat resistant film. When the fixing device is the film heating type, it is possible to use the heater (a heating member) with a low heat capacity at a high heating speed, and to use the heat resistant film with a small thickness. Accordingly, it is possible to increase a temperature of the components to a specific level in a short period of time. As a result, it is possible to reduce power consumption and a wet time (quick start) (Refer to Patent Reference).
Patent Reference: Japanese Patent Publication No. 2000-194211
In the conventional fixing device described above, the pressing member is pressed against the rotational member to form a nip portion therebetween for performing the fixing operation. When the pressing member is pressed against the rotational member at a high temperature for a prolonged period of time, the rotational member and an elastic layer of the pressing member tend to generate creep. In this case, when the printing operation is performed next time, a gloss variation tends to occur in an image.
In view of the problems described above, an object of the present invention is to provide an image forming apparatus capable of solving the problems of the conventional image forming apparatus.
Further objects and advantages of the invention will be apparent from the following description of the invention.